Thermoplastic polymers represented by organic polymers, particularly polyacrylic acid esters, polystyrene, polyethylene and the like are decomposed by heating and thus have heat decomposability and at the same time relatively high softening temperature and excellent for formability. Accordingly, the organic polymers have been used widely as binders for forming ceramics, thermal recording resins, thermally collapsing adhesives and the like on the basis of such heat decomposability and formability. On the other hand, recently, organic polymers in form of resin particles with micron sizes have been mixed with ceramics and fired to produce lightweight earthenware, glass filters and the like.
In such a manner, in the case resin particles of an organic polymer are used as a binder for ceramics or a lightening material, it is needed to remove the resin component by thermally decomposing or burning the resin particles by firing under the non-oxygen condition or the oxygen ambient.
However, in the firing process, the temperature at which the resin component is completely removed is close to the fusion temperature of ceramic raw material powder, glass powder or the like, residues such as carbon derived from resin particles have sometimes been left in the inside of a sintered body. Further, in the case the temperature at which the resin component is completely removed is higher than the fusion temperature, the sintered body has sometimes been deformed. Moreover, due to the combustion heat at the time of burning the resin component, deformation or cracking of a sintered body to be obtained has sometimes occurred. Accordingly, it has been desired to obtain resin particles whose resin component can be removed at a low temperature in the firing step and which scarcely causes strain by combustion heat and scarcely leave residues such as carbon derived from the resin component in the sintered body after firing.
To deal with such a problem, for example, Patent Document No. 1 discloses a thermally decomposable styrene type copolymer containing a styrene monomer and an α-methyl styrene monomer at a prescribed ratio as a resin material easy to be decomposed even at a low temperature and excellent in the molding processibility.
However, even such a thermally decomposable styrene type copolymer is still insufficient in the thermal decomposability at a low temperature and if it is tried to produce lighter earthenware using a large quantity of a resin, it is required to carry out the firing step at a high temperature not lower than 300° C. for a long time and therefore it takes a long time for the entire production step to result in a problem of lowering a production efficiency. Further, due to increase of combustion heat of the resin, a considerable strain is applied to a ceramic sintered body to be obtained and it results in a problem of occurrence of deformation and cracking. Accordingly, in the firing step, resin particles which can be decomposed thermally at a low temperature in a short time have been urgently required.
In recent years, a porous honeycomb filter wherein a partition wall of a honeycomb structure body obtained by sintering ceramic powder such as silicon carbide (SiC) or a cordierite powder was changed into the porous structure has been proposed. Such a porous ceramic filter (a porous honeycomb filter) is capable of removing various kinds of impurities contained in a fluid such as a gas by passing the fluid through the porous partitioning walls. The porous ceramic filter has been practically employed for such as a filter for trapping particulate of exhaust gas emitted from a diesel engine vehicle (a diesel particulate filter).
With respect to such a porous ceramic filter, the average fine pore diameter and porosity of the porous partitioning walls are very important factors for determining the functional property of the filter and in the case, for example, a diesel particulate filter or the like, a filter having a proper average fine pore diameter and high porosity has been required in accordance with the correlation among the particulate trapping efficiency, pressure loss, and trapping time.
Conventionally, as a method of controlling the average fine pore diameter and porosity of a porous ceramic material has been known a method of adding the pore-forming material such as graphite to a ceramic composition and degreasing and firing the obtained ceramic composition at a high temperature. However, this method requires use of a large quantity of a pore-forming material to obtain high porosity and in this case, the time of degreasing and firing carried out at 250° C. or higher is prolonged and thus the production step takes a long time and at the same time, considerably high strain is applied to the ceramic molded body due to increase of the combustion heat of the pore-forming material to result in a problem of occurrence of cracking in the molded body.
On the other hand, Patent Document No. 2 discloses a method of decreasing a combustion component by making the pore-forming material hollow to lower the combustion heat generation of the pore-forming material. According to the method, the combustion heat generation of the pore-forming material in the degreasing and firing step can be decreased and occurrence of cracking in the molded body can be lessened.
However, since combustion heat generation at 250° C. or higher still remains even in this method, in the case a porous ceramic material with a high porosity is to be produced by adding a large quantity of the pore-forming material, the problem of occurrence of cracking in the molded body is not solved yet.
Patent Document No. 1: Japanese Kokai Publication Hei-6-41241
Patent Document No. 2: Japanese Kokai Publication 2003-10617